Depth perception is the visual ability to perceive the world in three dimensions. Humans (and other animals) use a variety of monocular cues (that is, cues available from the input of just one eye) and binocular cues (that is, cues that require input from both eyes) to perceive depth.
Motion parallax is a type of monocular cue which affects depth perception. When an observer moves, the apparent relative motion of several stationary objects against a background gives hints about their relative distance. These subtle movements by the observer work in the real world for providing a better understanding of depth. However, when viewing images on a flat television or computer screen, such movements will not facilitate depth perception because there is no relative motion between objects shown in the two-dimensional image.
Stereopsis or retinal disparity is a type of binocular cue which affects depth perception. Information derived from different projection of objects on to each retina is used to judge depth. By using two images of the same scene obtained from slightly different angles, it is possible for the brain to calulate the distance to an object. If the object is far away, the retinal disparity will be small. On the other hand, if the object is close, the retinal disparity will be large. Again, this effect works in the real world to give a viewer a better understanding of depth, but does not work with a flat two dimensional screen because all objects on the screen appear to be at the same distance from the viewer.
Stereoptic effect may, however, be used to “trick” the brain into perceiving depth in a two dimensional image, such as a “Magic Eye” picture or a stereoscopic photo. Similarly, stereoptic effect may be used to produce a simulated three-dimensional image (that is, an image having depth cues) from a two-dimensional image such as an image on a flat television or computer screen, as described in European Patent Application Publication No. EP 1 636 631. This document describes an apparatus comprising a flexible Fresnel lens curved in two transverse directions so as to create a substantially convex lens. The apparatus may be mounted, for example, in front of a television screen or computer monitor 2 as shown in FIG. 1 to produce a simulated three-dimensional image of the two-dimensional image displayed on the screen. The Fresnel lens 3 is spaced apart from the screen 2 and is curved in a first plane (the x-z plane) and a second plane (the y-z plane) so as to form a Fresnel lens having two planes of curvature. As shown in the drawing, if a cross-section of the lens were taken in the x-z plane, the cross-section of the lens would be curved or arcuate in shape across its entire width. Similarly, if a cross-section were taken in the y-z plane, the cross-section would also be curved or arcuate in shape. As the x- and y-planes are, by definition, orthogonal to one another, there are thus two orthogonal planes in which the lens is arcuate in cross-section. The Fresnel lens may be flexible and positioned within a mount configured with adjustable tensioning members so as to tune the optical characteristics of the Fresnel lens so as to optimise production of the simulated three-dimensional image.
Because the Fresnel lens is curved in two transverse planes, slightly different images are received by the left and right eyes of the viewer, producing a stereoptic effect, which is interpreted by the brain so that the image appears to have depth, that is, the image appears more three-dimensional than would otherwise be the case.
In EP 1 636 631, the corners of the Fresnel lens (for a rectangular lens) are fixed in position in order to achieve the required curvatures. Stresses are introduced into the curved Fresnel lens at the fixed points and this also produces stresses in other parts of the lens including along the edges of the lens between the fixed points. A device as described in the above patent application will produce a distorted image at these stress zones including many zones of the overall image. For example, in the case of a rectangular screen, the four corners—the areas of the screen furthest away from the viewer—will show most pronounced distortions, and the four edges will show a “bowing” effect. The image in these areas will not be straight but will bow outwards away from the centre of the Fresnel lens as shown in FIG. 2. In reality, the bowing effect may be even more pronounced, and may extend over a greater portion of the screen than in the example shown in FIG. 2.
The present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.